Emergent smectic order in simple active particle models

TL;DR

This paper reveals the emergence of smectic order in simple active particle models, showing how particles form and move along rows with different instabilities and phases, supported by hydrodynamic theory.

Contribution

It introduces the novel smectic-P phase in active particles and develops a hydrodynamic theory explaining their behavior and instabilities.

Findings

Smectic order decays algebraically with system size up to a finite scale.

A undulation instability limits smectic order in apolar models.

Traveling fluctuations disrupt rows and cause global rotation in polar models.

Abstract

Novel "smectic-P" behavior, in which self-propelled particles form rows and move on average along them, occurs generically within the orientationally-ordered phase of simple models that we simulate. Both apolar (head-tail symmetric) and polar (head-tail asymmetric) models with aligning and repulsive interactions exhibit slow algebraic decay of smectic order with system size up to some finite length scale, after which faster decay occurs. In the apolar case, this scale is that of an undulation instability of the rows. In the polar case, this instability is absent, but traveling fluctuations disrupt the rows in large systems and motion and smectic order may spontaneously globally rotate. These observations agree with a new hydrodynamic theory which we present here. Variants of our models also exhibit active smectic "A" and "C" order, with motion orthogonal and oblique to the layers respectively.